It's a mature bull market, so investors should continue to invest in U.S. stocks that continue to show signs of strength.

Goldilocks Economy in the U.S.

The U.S. economy is moving along at a moderate pace and is expected to continue its stable upswing in 2015. Here are some indicators that illustrate why:

• The Index of Consumer Sentiment from University of Michigan rose above its long term average of 80; • Weekly Initial Unemployment Claims are near 40-year lows; • Housing Starts are improving; • ISM Manufacturing PMI Composite Index is staying well above 50 level; • Low inflation expectations from falling oil prices and record expansion in solar utility industry.

Corporate earnings growth, the most reliable market gauge in the U.S., is forecasting 10% growth in 2015.

Look to Earnings

Investors should focus on companies that can increase the most earnings per share efficiently while maintaining low valuation and not be afraid to increase cash allocation.

With the U.S. dollar strengthening, lower energy prices and higher consumer spending, the best bets would be on growth at reasonable priced stocks, from consumer discretionary and information technology sectors. With 10,000 baby boomers turning 65 and eligible for Medicare every day and the costly implementation of Obamacare, demands and innovations in the health care sector will create tremendous investment opportunities.

Here's one stock to consider in the large-cap growth space: Western Digital (WDC - Get Report) . WDC's one-year earnings per share growth rate is 64% and the dividend increased by 46% this year versus last.

No Bears in the U.S. in 2015

To quote Sir John Templeton, "Bull markets are born in pessimism, grow on skepticism, mature on optimism and die on euphoria."

Investor sentiment indicators, such as the AAII Investor Sentiment Survey, continue to show a narrow bull-bear spread, indicating that general investors are still skeptical about this five-and-a-half-year-old bull market. Investors and traders continue to embrace momentum trades, creating volatile yet profitable market condition. Bears will probably come out of hibernation when broad-based investors turn to buy-and-hope strategies again and become euphoric about the stock market.

Until then, as Warren Buffett said, "Look at market fluctuations as your friend rather than your enemy; profit from folly rather than participate in it."

TheStreet Ratings team rates WESTERN DIGITAL CORP as a Buy with a ratings score of A-. TheStreet Ratings Team has this to say about their recommendation:

"We rate WESTERN DIGITAL CORP (WDC) a BUY. This is based on the convergence of positive investment measures, which should help this stock outperform the majority of stocks that we rate. The company's strengths can be seen in multiple areas, such as its solid stock price performance, revenue growth, largely solid financial position with reasonable debt levels by most measures, attractive valuation levels and good cash flow from operations. We feel these strengths outweigh the fact that the company has had sub par growth in net income."

Palo Alto Networks' crucial earnings day events on Wednesday could determine share price direction in the immediate term. Ahead of the print, the stock still looks like a solid GARP play to be held over longer periods of time.